Generally, conventional power strips are susceptible to transient surge voltages resulting from lightning strike. For preventing the transient surge voltages from damaging the electrical appliances which are electrically connected to the power strip, the conventional power strip usually has a surge protective circuit for preventing damage of electrical appliances.
Recently, as the requirements for electrical safety of electrical appliances become more stringent, stringent safety regulations associated with surge protective circuit are provided. Underwriters Laboratories Standard for Safety for Transient Voltage (UL 1449) is the primary safety standard for testing surge protective circuit. The second edition of UL 1449 proposes a lightning strike simulation test. In accordance with the lightning strike simulation test, when a voltage of 6,000 volts and a current of 500 amperes are inputted into an electrical appliance having a surge protective circuit, the surge protective circuit should exhibit a clamping voltage less than 330 volts.
On Sep. 29, 2006, UL 1449 has recently been revised, and will be introduced as a third edition. In accordance with the lightning strike simulation test proposed in the third edition of UL 1449, the surge protective circuit should also exhibit a clamping voltage less than 330 volts even if the test condition become more stringent (i.e. 6,000 volts, 3,000 amperes). Experiments demonstrate that most surge protective circuits meeting the requirements of the second edition of UL 1449 exhibit a clamping voltage larger than 400 volts. In other words, the surge protective circuits of the conventional power strips fail to meet the requirements of the third edition of UL 1449.
For meeting the requirements of the third edition of UL 1449, a power strip with a multi-stage surge protective circuit has been disclosed. FIG. 1 is a schematic view illustrating a power strip 10 with a multi-stage surge protective circuit 1031 according to the prior art. The power strip 10 further includes a plug device 101, a conducting line 102 and a power strip main body 103. The plug device 101 includes three pins 1011. The power strip main body 103 includes the surge protective circuit 1031 and a plurality of electrical sockets 1032.
When a surge occurs, the surge is inputted into the plug device 101 through any of the three pins 1011 and then transmitted to the power strip main body 103 through the conducting line 102. After the surge is suppressed by the surge protective circuit 1031 in multiple stages, the electricity will be transmitted to the electrical sockets 1032 so as to protect the electrical appliances which are electrically connected to the electrical sockets 1032.
Please refer to FIG. 1 again. The surge protective circuit 1031 includes a first surge protective device 10311, an inductor 10312 and a second surge protective device 10313. The first surge protective device 10311 and the second surge protective device 10313 are used to suppress the surge for the first and second times, respectively. The inductor 10312 is used for conducting the surge. Examples of the first surge protective device 10311 and the second surge protective device 10313 include metal oxide varistors (MOVs), gas discharge tubes (GDTs) or transient voltage suppressors (TVSs). The surge protective circuit 1031 used in the power strip 10, however, still has some drawbacks. For example, the inductor 10312 is not cost-effective and the volume thereof is bulky. Correspondingly, the power strip 10 has high fabricating cost and a large overall volume.
Therefore, there is a need of providing a power strip having a surge protective circuit to meet the requirements of the third edition of UL 1449 while having the advantages of small size, cost-effectiveness and industrial utility.